1. Field of the Invention
This invention relates to a new electromagnet-striker bar system for a rotary dobby control device which, by considerably reducing wear between the ferrous material striker bars and the relative amagnetic support and slide shoulders and always ensuring a constant optimum predetermined value for the air gap between the electromagnets and said striker bars, results not only in large constructional economy but also in considerable operational safety and reliability with time, as required by modern high-speed rotary dobbies.
More precisely, the invention relates to an improvement in the electromagnet packs used in the two identical control units of the rotary dobby control device, as described in European patent application publication No. 0525862 in the name of the present applicant.
2. Discussion of Background
As known from the above European application, each control unit comprises a set of striker bars of ferrous material equal in number to the number of main crank arms in the dobby, which can be twenty or more in number, each bar being urged by its individual spring against an accompanying arm which rotates it from a first position in which it cooperates with the control pusher of its individual operating unit to a second position in which it cooperates with its individual electromagnet energized in accordance with a predetermined program. All of the striker bars are hinged at one of their ends to their individual arm projecting from a single shaft which rocks them in a direction perpendicular to this shaft. To prevent this rocking of the bars causing undesirable sliding or deleterious contact of the bars against their electromagnets when the bars are retained in the second position by the respective energized electromagnets, each striker bar, when in the second position, is in contact with an amagnetic support and slide shoulder interposed between the pole pieces of the electromagnets in order to create a very small air gap between the bar and the relative electromagnet.
In this known construction problems have arisen due substantially to too rapid wear of the sliding region between each striker bar and the relative amagnetic shoulder and to the considerable difficulty of maintaining all the striker bars attracted by the relative electromagnets as far as a precise and very small distance from the pole pieces of the electromagnets.
In this respect, as the amagnetic support and slide shoulders are made to act centrally between the pole pieces of each electromagnet, there is no possibility of hardening the contact surfaces to reduce wear, given that any heat treatment of the striker bars would generate a residual magnetism in the bars, compromising proper operation of the dobby in that the residual magnetism would maintain the bars attracted to the relative shoulders even after the energization current of the relative electromagnets is cut off.
Again, for perfect operation the striker bar attracted by the relative electromagnet must always be maintained at a precise optimum minimum distance from the pole pieces of the electromagnet, i.e. a minimum air gap must be created which has been found experimentally to be of the order of about 0.2 millimeters. In this respect a greater minimum air gap would drastically reduce the magnetic attraction of the electromagnet, with the danger that this latter attraction is no longer able to overcome the action of the reaction spring of the striker bar, which would therefore be withdrawn from the relative shoulder even though the electromagnet is energized.
A zero air gap is also deleterious and disastrous in that a residual magnetism would be created in the striker bar of such intensity that the reaction spring would no longer be able to detach the bar from the relative shoulder when the electromagnet is de-energized.
Finally a minimum air gap less than the optimum value would result in a very large force of attraction with consequent increase in wear.
As currently the electromagnet pack is generally formed by stacking the relative magnetic cores using two locking ties which pass through holes in the cores and are locked at their ends by shoulder bolts to form a structure which is not perfectly rigid, it is clearly extremely difficult if not impossible to maintain the predetermined value for the air gap with time in the case of all the pairs of striker bars and electromagnets, which can be twenty or more in number.
Again, the inevitable constructional imperfections of the various constituent elements of the electromagnet pack make it even more difficult and costly to achieve the very small air gap.